How do I enter Frontier Caverns?

Okay, I was trying to avoid having to do this; but sadly the game is too new and I can't really find any information on it. So if any other player has information, I'd greatly appreciate it.

I'm currently on my New game plus/ 2nd playthrough.I've only progressed about as far as the Wyrm Hunt License on this new playthrough, and I didn't really bother to seek out the Caverns on my first playthrough, so I'm not sure if it unlocks after a certain point in story.

However, I was trying to find a Hydra so that I can enchant an armor to tier 3 (shoulda just DF'ed it, but I used a dif armor)

I got the Achievement "Entered Frontier Cavern" or w/e; but the actual gate in that tunnel is locked. I can see the big room with a few goblins, but it's locked. Examining the gate gets "There's aught there" or something. Is this gate into the true Frontier Caverns only opened by doing something specific?

I mean, the loading screen has the tip that says something like how the monsters in Frontier Caverns were strong.And I've ran across all manner of beasts; even the Drake, and even spent some time knicking off some of the Ur-Dragon's HP; yet I've been inable to find a Hydra; save the story battle in Encampment.

So, if anyone can tell me how to get into Frontier Caverns within Shadow Fort; or tell me where to find a Hydra; I would be much obliged.

Marked for spoilers for just in case; although is really minor in my opinion.

Accepted Answer

After you do the story quest there you can pick up a quest in the capital called "Put The Eye Out". After that once you go into the Shadow Fort fall through that same hole you did, but this time you'll have access to the Frontier Caverns and get to go in the big room to fight an armored cyclops.